Exam 22: Inheritance Of Complex Traits

Correct Answer:

Verified

Nilsson-Ehle studied color variation in wheat grains.When he crossed a white-grained variety with a dark red-grained variety,he obtained an F₁with an intermediate red phenotype (Figure 23.1).Self-fertilization of the F₁ produced an F₂ with seven distinct classes,ranging from white to dark red.The number of F₂ classes and the phenotypic ratio that Nilsson-Ehle observed suggested that three independently assorting genes were involved in the determination of grain color.Nilsson-Ehle hypothesized that each gene had two alleles,one causing red grain color and the other white grain color,and that the alleles for red grain color contributed to pigment intensity in an additive fashion.Based on this hypothesis,the genotype of the white-grained parent could be represented as aa bb cc,and the genotype of the red-grained parent could be represented as AA BB CC.The F₁ genotype would be Aa Bb Cc,and the F₂ would contain an array of genotypes that would differ in the number of pigment-contributing,alleles present.Each phenotypic class in the F₂ would carry a different number of these pigment-contributing alleles.The white class,for example,would carry none,the intermediate red class would carry three,and the dark red class would carry six.Nilsson-Ehle's work,published in 1909,showed that a complex inheritance pattern could be explained by the segregation and assortment of multiple genes.
The American geneticist Edward M.East extended Nilsson-Ehle's studies to a trait that did not show simple Mendelian ratios in the F₂.East studied the length of the corolla in tobacco flowers (Figure 23.2a).In one pure line,the corolla length averaged 41 mm; in another,it averaged 93 mm.Within each pure line,East observed some phenotypic variation-presumably the result of environmental influences (Figure 23.2b).By crossing the two lines,East obtained an F₁ that had intermediate corolla length and approximately the same amount of variation that he had seen within each of the parental strains.When East intercrossed the F₁ plants,he obtained an F₂ with about the same corolla length,on average,that he saw in the F₁; however,the F₂ plants were much more variable than the F₁.This variability was due to two sources: (1)the segregation and independent assortment of different pairs of alleles controlling corolla length,and (2)environmental factors.East inbred some of the F₂ plants to produce an F₃and observed less variation within the different F₃ lines than in the F₂.The reduced amount of variation within the F₃ lines was presumably due to the segregation of fewer allelic differences.Thus,the complex inheritance pattern that East observed with corolla length could be explained by a combination of genetic segregation and environmental influences.